Bingzhe Bai

Effect of tempering temperature and carbide free bainite on the mechanical characteristics of a high strength low alloy steel

Abstract The effect of tempering temperatures and the role of carbide free bainite (CFB) on the mechanical properties and susceptibility to hydrogen embrittlement of a low carbon Mn–Si–Cr steel have experimentally been investigated. The air-cooled steel consists of a mixed microstructure of carbide free bainite–martensite (CFB/M), which exhibits a combination of strength and toughness superior to a water-quenched martensite microstructure, after intermediate temperature tempering. The susceptibility to hydrogen embrittlement of the steel with the CFB/M microstructure decreases as the tempering temperature increases from 280 to 370xa0°C. This is attributed to the CFB/M mixed microstructure which not only possesses high temper resistance but also delays the tempered martensite embrittlement onset temperature. Films of carbon enriched retained austenite in the CFB exhibit good mechanical stability which enhances the toughness of the steel.

Performance comparison of AlTiC and AlTiB master alloys in grain refinement of commercial and high purity aluminum

Abstract For further knowledge about the refining performance of AlTiC master alloys, A15.5Ti0.25C and A16.5Ti0.5C master alloys containing high Ti and C content were prepared and used in grain refining experiments of 99.8% commercial pure aluminum(CPAl). Their performance was compared with two types of Al5Ti1B refiners whose performance was nowadays considered to be the best. These two types of master alloys show similar refining efficiency at the addition level of 0.2%. However, at the addition level of 0.5%, there still exists great performance difference between AlTiC and Al5TiB alloys in grain refinement of 99.98% and 99.995% high purity aluminum(HPAl). The growth of columnar grains is fully suppressed due to the refinement of AlTiC at the addition level of 0.5%. Also, at the same addition level, the grain refining experiments of A13Ti0.15C and A15Ti0.2C master alloys which have found initial commercial applications are conducted in the above-mentioned three types of pure aluminum. According to the experimental results, these two refiners of different compositions are both nonideal. The second phase particles extracted from each refiner were observed through TEM, while the nuclei of grains after grain refinement were observed through SEM. The results were analyzed through computation and comparison of the constitutional-supercooling parameter and the growth-restriction parameter whose values were determined by solute element in aluminum melt with different purity. Apparently, AlTiC master alloys with high content of Ti and C element have great refining potential.

Mn-Series Low-Carbon Air-Cooled Bainitic Steel Containing Niobium of 0.02%

A new hot rolled low-carbon air-cooled bainitic steel containing Nb of 0.02% has been developed based on alloying design of the grain boundary allotriomorphic ferrite (FGBA)/granular bainite (BG) duplex steel. The as-rolled microstructure and mechanical properties of bainitic steel containing Nb of 0.02% were investigated by tensile test, optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that adding 0.02% of Nb obviously improves the strength without sacrificing toughness of the FGBA/BG steel. Adding 0.02% of Nb not only refines the grain boundary allotriomorphic ferrite grains but also promotes the refinement of granular bainite including its bainitic ferrite and M/A island. Any Nb(C, N) has been hardly observed in the steel containing Nb of 0.02%. It is suggested that the strengthening mechanism of Nb of 0.02% can be mainly attributed to the effect of Nb in solution (solute drag-like effect) on the phase transformation rather than the precipitation strengthening of Nb(C, N) particles.

CCT Curves of Low-Carbon Mn-Si Steels and Development of Water-Cooled Bainitic Steels

CCT curves of Mn-Si steels with different manganese contents or carbon contents were determined. The results show that the transformation range of bainite can be separated from that of ferrite when the manganese content approaches a certain content, and the incubation period of ferrite increases more significantly than that of bainite transformation with the increase of carbon content in Mn-Si steels. Furthermore, water-cooled bainitic steels without adding expensive alloying element were developed. Granular bainite was obtained when a bar with diameter of 300 mm was cooled by water, and a mixed microstructure of granular bainite and martensite was obtained in water-cooled plate with thickness of 40 mm. The developed water-cooled bainitic steels containing no expensive alloying element showed a good combination of strength and toughness. The tensile strength, yield strength, and toughness (AKU at −20 °C ) of bar with diameter of 300 mm after water cooling were higher than 850 MPa, 620 MPa, and 65 J, respectively, and those of plate with thickness of 40 mm after water cooling were higher than 1000 MPa, 800 MPa, and 50J, respectively.

A Mn-Series of Oil-Quenched Super-Strength Bainitic Steel With High Hardenability

A new Mn-series of oil-quenched super-strength bainitic steel was developed on the basis of the alloy system of the original Mn-series air cooled bainitic steel. The mechanical properties of the new steel are well combined while precious alloy elements and complicated technologies are not needed for its production. Experimental results show that bainite/martensite (B/M) dual phase microstructure can be obtained within a very wide cooling rate range and bainite microstructure exists even when the cooling rate reaches as fast as 60 C/s; the hardenability of the steel is better than that of 40CrNiMo and 42CrMo; the diameter of the round bar made of the steel is 250 mm, and the round bar tempered at 350 °C after oil quenching exhibits its central mechanical properties as follows: Rm = 1550 MPa, Rp0.2 = 1270 MPa, A = 15%, aku = 95 J/cm2 (at room temperature).

RBF-Type Artificial Neural Network Model Applied in Alloy Design of Steels

RBF model, a new type of artificial neural network model was developed to design the content of carbon in low-alloy engineering steels. The errors of the ANN model are: MSE 0.0521, MSRE 17.85%, and VOF 1.9329. The results obtained are satisfactory. The method is a powerful aid for designing new steels.

Influence of Nb on Microstructure and Property of Low-Carbon Mn-Series Air-Cooled Bainitic Steel

The effect of Nb on the microstructure evolution of low-carbon Mn-series air cooled bainitic steels has been studied by using thermomechanical simulation machine, SEM, and TEM observation. The results show that the amount of ferrites is relatively higher in the steel without Nb than that of Nb-bearing steel under same deformation conditions, and the ferrites in Nb-bearing steel are more finer. The mechanical properties of the Nb-bearing steel are higher than those of the steel without Nb at the same finishing rolling temperature (FRT), and the toughness of the Nb-bearing steel is about 100 J higher than that of the steel without Nb at the finishing rolling temperature 750 °C. The toughness of Nb microalloyed steel will be improved more effectively than that of the steel without Nb with the decrease of FRT.

Effects of Nb on the Microstructure and Mechanical Properties of Water-Quenched FGBA/BG Steels

In order to reduce the alloying cost, Mn-series low carbon water-quenched grain boundary allotriomorphic ferrite (FGBA)/granular bainite (BG) steels have been developed. The effect of 0.06xa0wt.% Nb on microstructure and mechanical properties of FGBA/BG steel was investigated. The result showed that the addition of 0.06xa0wt.% Nb improved the hardenability of the FGBA/BG steel, refined the grain size of FGBA, promoted the granular bainitic transformation, and refined the granular bainite including its bainitic ferrite and martensite/austenite (M/A) constituents. With the addition of 0.06xa0wt.% Nb, the yield strength increased from 560 to 741xa0MPa, and the impact energy increased from 93 to 151xa0J, respectively, for 30-mm thickness steel plates. It is supposed that the addition of 0.06xa0wt.% Nb could improve the mechanical properties of the FGBA/BG steel by refining the microstructure and increasing the amount of strengthening phases.

Effect of Microstructure on Corrosion Fatigue Behavior of 1500 MPa Level Carbide-Free Bainite/Martensite Dual-Phase High Strength Steel

Influence of microstructure of the experimental steels on the corrosion fatigue behavior in 3. 5% of NaCl aqueous solution was studied. Experimental results show that compared with the full martensite (FM) steel, the car-bide-free bainite/martensite (CFB/M) steel has higher corrosion fatigue strength and corrosion fatigue crack threshold (∆Kthef), and lower corrosion crack propagation rate [Cda/dN)ef].

Effect of Thermomechanical Treatment Temperature on Structure and Properties of CFB/M Ultra-High Strength Steel

Modified CCT diagram of carbide-free bainite-martensite (CFB/M) ultra-high strength steel was established by applying controlled cooling of small samples. In addition, the influence of thermomechanical treatment temperature on the structure and properties was discussed. The experimental results showed that when deformed at 860 °C and below, ferrite transformation occurred due to strain. With the decrease of ausforming temperature, the quantity of ferrite increased and strength and toughness were deteriorated. Therefore, certain information was provided for optimizing technical parameter of ausforming process; firstly, the thermomechanical treatment temperature should not be lower than 860 °C in order to avoid ferrite formation induced by deformation; secondly, rapid cooling rate is also significant after deformation in order to avoid ferrite precipitation during subsequent cooling stage.